Protective coatings by chromium diffusion of metal parts and process

ABSTRACT

The invention relates to the preparation of a protective coating by diffusion of chromium and iron in combination into the surface of a metal base in which the surface is formed of nickel or an alloy of nickel.

United States Patent Karl Bungardt Kreteld;

Gottfried Becker, Dusseldorf; Gunter Lehnert; Gustav Lennartz, both of Kreield,

[72] Inventors all of Germany [211 App]. No. 607,148

[22] Filed Jan. 4, 1967 [45] Patented Jan. 4, 1972 [73] Assignee Deutsche Edelstahlwerke Aktiengesellsehaft Krefeld, Germany [32] Priority Jan. 5, 1966 [33] Germany [54] PROTECTIVE COATINGS BY CHROMIUM DIFFUSION OF METAL PARTS AND PROCESS 7 Claims, No Drawings [52] US. Cl ..117/l07.2R,

Primary ExaminerAlfred L. Leavitt Assistant Examiner-Alan Grimaldi Attorney-McDougall, Hersh, Scott & Ladd ABSTRACT: The invention relates to the preparation of a protective coating by diffusion of chromium and iron in combination into the surface of a metal base in which the surface is formed of nickel or an alloy of nickel.

PROTECTIVE COATINGS BY CHROMIUM DIFFUSION OF METAL PARTS AND PROCESS The invention refers to the production of protective coats by diffusion of metal on parts which show a high content of nickel at the surface. These coats are relatively thick and show oxidation, corrosion, thermal shock and crack resistance, especially at high temperatures.

It is well known that nickel has a high resistance against atmospheric corrosion. That is why metal parts have been coated with nickel through a galvanic process, for example. However, the nickel is attacked while heating up by oxidation or corrosion comparatively early, depending on the surrounding atmosphere, more or less, by air, oxygen, sulphur or sulphur combinations.

It is furthermore known that an addition of 20 to 60 percent chromium to the alloy can strengthen the resistance against oxidation and corrosion attacks, especially at higher temperatures, but not in every case is an alloy wanted of 60 percent chromium and 40 percent nickel. Since especially the surface is exposed to oxidation or corrosion attacks, a protective coat of nickel and chromium would be sufiicient.

Experiments which were made in this direction to diffuse chromium into a high nickel containing surface showed that the chromium concentration decreased from the surface towards the center of the part. These coats showed a chromium concentration necessary for the resistance in a small part of the zone. This concentration lies at about 50 percent and does not always fulfill the requirements. For reasons of simplicity, some test were undertaken on samples of pure nickel and also on samples of nickel alloys with about 75 percent nickel, 20 percent chromium, and the remainder of different elements. Each case resulted in a comparatively fast decrease of the chromium concentration from the surface toward the center of the samples.

With this invention a new process is to be found which allows the formation of thicker protective coats through diffusion of chromium into such surface. These coats should contain a rather high chromium concentration on a metal part which has a high nickel content at the surface, especially on parts with a nickel coat or parts from nickel or nickel alloys.

To answer these questions the inventors suggest to diffuse iron together with the chromium.

Parts which were coated with nickel, as well as those from pure nickel or at least 50 percent nickel containing alloys, which were treated according to the invented process showed a surprisingly thick coat with a high chromium concentration. However, no deterioration of these attributes of the produced coat showed up as normally should be expected. It was also surprising that the indiffused iron did not take any space of the chromium indiffused at the same time. On the contrary the nickel content at the surface was lowered at a percentage equivalent to the amount of iron. This shows a further advantage because the melting point of the eutecticum ferro-ferrosulfide (Fe-FeS) lies at 988 C., while the eutecticum nickelnickel sulfide (Ni-Ni S is at 645 C. This way the resistance against high temperatures is increased.

The transformation or indiffusion of chromium and iron takes place through the vapor phase where the chromium as well as the iron are existing as a halogen combination. The parts are formed of a base of nickel or a nickel alloy or also of a metal or metal alloy which were previously coated with nickel with a melting point above the diffusion temperature. These samples to be treated are suitably packed in a mixture of 10 percent chromium chloride, 5 percent iron chloride and percent aluminum oxide, and are heated according to the coat thickness to l,000 -l,200 C. for 5 to 20 hours. The indicated combination of the packing mixture is only an example. Instead of the chloride other halides can be used for the same purpose according to the invention.

In addition to the parts made from pure nickel or nickel alloys with 50 percent nickel and melting points above 1,100" O, the invented process is also used at the advantage with metals and metal alloys which do not contain enough nickel to make possible the buildup of the wanted chromium-nickeliron coat. It is also advantageous with metals which cannot be chromized, because, for example, a limitation of diffusion exists through the material in connection with a mixed crystal buildup or the solubility for chromium or the hindering through other elements such as carbides or equivalents. In such cases, a nickel coat is put on through galvanic processes or spraying to produce a base in which the chromium and iron can be diffused. This process has the advantage for parts from metals or metal alloys especially when the center of the part should show high hardness, stress resistance, or other special attributes, and when the surface should be protected against corrosion, crack build up or thermal shock stress at high temperatures.

We claim:

1. The process of the production of a protective coating on a base surface formed of nickel or an alloy of nickel comprising the steps of exposing the surface to the vapors of a chromium halide and iron halide in the presence of aluminum oxide at a temperature above 1 ,000 C.

2. The process as claimed in claim 1 in which the base metal is packed in a mixture of chromium halide, iron halide and aluminum oxide and heated to a temperature of l,000-1,200 C. for more than 5 hours.

3. The process as claimed in claim 2 in which the materials are present in the pack in the ratio of about 10 parts by weight chromium halide, 5 parts by weight iron halide and 85 parts by weight aluminum oxide.

4. The process as claimed in claim 2 in which the chromium halide is chromium chloride and in which the iron halide is iron chloride.

5. The process as claimed in claim 2 in which the materials are heated at diffusion temperature for a time within the range of 5 to 20 hours.

6. The process as claimed in claim 2 in which the metal base is of a metal having a coating on the surface formed of nickel or an alloy of nickel.

7. A metal base having a surface of nickel or an alloy of nickel and a diffusion coating in the surface prepared by the process of claim 1. 

2. The process as claimed in claim 1 in which the base metal is packed in a mixture of chromium halide, iron halide and aluminum oxide and heated to a temperature of 1,000*-1,200* C. for more than 5 hours.
 3. The process as claimed in claim 2 in which the materials are present in the pack in the ratio of about 10 parts by weight chromium halide, 5 parts by weight iron halide and 85 parts by weight aluminum oxide.
 4. The process as claimed in claim 2 in which the chromium halide is chromium chloride and in which the iron halide is iron chloride.
 5. The process as claimed in claim 2 in which the materials are heated at diffusion temperature for a time within the range of 5 to 20 hours.
 6. The process as claimed in claim 2 in which the metal base is of a metal having a coating on the surface formed of nickel or an alloy of nickel.
 7. A metal base having a surface of nickel or an alloy of nickel and a diffusion coating in the surface prepared by the process of claim
 1. 